216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



solved in the ground water that they can be transported from place 

 to place, and it is only as they occur in solution that they have a 

 harmful effect on the growth of plants or on the physical character 

 of the soil. For these reasons the alkali problem is always a problem 

 of the ground water or the soil solution. 



To understand the alkali problem one must understand something 

 of the conditions which influence the movements and the reactions 

 of the soil solution in relation to the soil. Water exists in and moves 

 through the soil in the liquid form and as a vapor. It is only in the 

 liquid form that it is capable of dissolving and transporting salts. 



The water content of the soil varies within wide limits. A satur- 

 ated soil may hold a quantity of water equal to half its own weight. 

 A soil that is thought of as being dry when examined in the field 

 may have as much as 8 or 9 per cent of water, in case of a very 

 fine soil, or not more than 2 or 3 per cent of water in the case of a 

 sandy soil. When in good condition to support plant growth, ordi- 

 nary soils contain from 10 to 30 per cent of water. These figures are 

 iiiven to show that if the water contained in a certain quantity of 

 soil has dissolved in it a certain quantity of salts, the concentration 

 of the solution may vary within wide limits as the soil approaches the 

 air-dry limit on the one hand or the saturation limit on the other. 



It is probably because of this variation in the moisture content 

 of the soil that it is customary to refer to the proportion of alkali 

 salts as a percentage of the dry weight of the soil rather than to 

 speak of the concentration of the soil solution. Thus in classifying 

 or mapping alkali soils, it is the custom to describe the different 

 areas as containing 1 per cent or 2 per cent of salts, meaning by this 

 that the proportion of water soluble material is equal to 1 per cent 

 or 2 per cent of the dry weight of the soil. 



The concentration of the salts in the soil solution is a very differ- 

 ent thing from the percentage of salts in the soil. Thus if a soil 

 contained 1 per cent of soluble salts and 25 per cent of water, the 

 concentration of the soil solution would be equivalent to 4 per cent, 

 while if the same soil were merely permitted to dry out until it con- 

 tained only 10 per cent of water, the concentration of the soil solu- 

 tion would be equivalent to 10 per cent. These figures give some- 

 thing of an idea of the concentrations of salt that may be tolerated by 

 certain desert plants which are able through special adaptations to 

 grow in soils containing as much as 3 per cent of soluble salts, 

 and which are subject to periods of drought during which the mois- 

 ture content may be reduced to 10 per cent or less. 



It has been noted above that the soluble salts are dissolved in and 

 move with the liquid water of the soil, and that when the soil solu- 

 tion is evaporated from the surface of the soil, the salts are left be- 



